[2.1] split part of multi-geometry

This adds a new advanced map tool to split multi-geometry features.

The geometry class has been modified:
- 3 addPart methods: on for list of points, one for QgsGeometry and one for GEOSGeometry
- instead of checking that polygons are disjoint, it checks if they are not overlapping (once split, the features are not disjoint but they do not overlap)